1. Field of the Invention
The invention pertains to a laser spot scanning system for communicating information to a scanned medium and especially to a scanning system which utilizes reflected light from a multifaceted rotating polygon.
In particular, the invention is directed to a spot scanning system using acousto-optical and electro-optical means and methods to compensate for defects in facet to facet relationship and facet to axis error.
2. Description of the Prior Art
A recurring problem in scanner systems is to reduce or eliminate error introduced as a result of inherent defects in the construction of rotating polygonal mirrors. Such defects occur usually in the angular relationship between adjacent facets (facet-to-facet) and between facet planes and the polygonal rotational axis (facet-to-axis). A typical solution is to employ nonspherical optics to partially correct the effects of facet angular error as is shown in the apparatus described in the U.S. Pat. No. 4,002,830. Another situation is the use of optical reflecting or refracting elements pivotally mounted in the path of radiation utilizing electromechanical devices that are energized by timed electrical signals such that the refracting element is pivoted to correct the scanning errors caused by angular defects in the rotating mirror. The control of electromechanical devices is preprogrammed to make proper adjustments and the fabrication of these optical systems is thus expensive, as is alignment of the same.
In contrast to systems which use an encoder and logic to correct for error and operate in an open-loop manner by applying a predetermined correction factor, the instant system employs sensing optics and a feedback loop to correct the position of the raster scan lines.